1.1 Field of the Invention
This invention is in the field of missile guidance and relates to a device which senses, in real time, the boresight error slope.
1.2 The Prior Art
In a missile which employs a terminal homing seeker and a proportional navigation guidance law, the space rate of change of boresight error, i.e., the boresight error slope, is one of the predominant error sources. This slope is defined as a small change in boresight error divided by a small change in aspect angle. With a proportional navigation guidance law, it is required that the line-of sight (LOS) to the target not rotate in inertial space. Thus an error in line-of sight rate rather than an error in LOS angle, per se, is the predominamt error. When the boresight error slope (denoted by m) is multiplied by body rate (denoted by .theta.) an error in LOS rate (denoted by .DELTA..beta.) is produced. Since .DELTA..beta. is in a parasitic loop from body rate to apparent target motion, through the guidance gain, and back to body rate it can cause erratic instabilities.
Various approaches have been used to minimize either the boresight error slope or its effect on missile guidance. These include:
a. Reducing guidance loop gain or increasing guidance time constant. This compromises guidance accuracy.
b. Controlling radome wall thickness during the fabrication process by machine grinding or forming. This is expensive, time consuming, and usually yields a boresight error slope greater than about 0.06 degrees per degree.
c. Preflight mapping the boresight errors, storing these errors in a look-up table and actively compensating for the errors during flight. Although residual errors after compensation have been measured as low as 0.01 deg/deg this is very expensive since each radome must be individually mapped. Also, this does not compensate for inflight variation of errors.
d. Opening the guidance loop and introducing a known dither, in both pitch and yaw, of the body axis about the velocity vector while the seeker is still tracking the target. The measured LOS rate is then compared with that expected from the known dither rate to obtain the LOS rate error. This technique may introduce oscillation into an otherwise marginally stable missile. It takes considerable time and energy because of the two-axis dither. The dither is necessarily slow because of missile response time; therefore the data may not be in real time for hypersonic flight where the radome statistics are changing rapidly. This method has never been tested.
It has been found that for supersonic flight at high altitude with low aerodynamic q, a boresight error slope (m)&lt;0.01 deg/deg is required to prevent the parasitic loop from causing the missile to go unstable. Thus the foregoing approaches to reducing m may not be satisfactory.